This is a nationalization of PCT/BE00/00095 filed Aug. 21, 2000 and published in English.
The present invention relates to a process for a direct hydrolysis of a fatty acid ester to the corresponding fatty acid in the presence of a catalyst, as described in the preamble of the first claim.
Today fatty acids, in particular those fatty acids containing 6 to 20 carbon atoms, play a key role in modem technology and find numerous applications in lubricant formulations, food additives, plasticizers, etc. Thereby the fatty acids can either be used as single components or as a mixture of well-defined fatty acids. The main sources for the production of fatty acids are the native oils or tri-glycerides.
From GB-A-2.146.638 a process is known for the production of fatty acids starting from the corresponding lower fatty acid alcohol esters. In the process described in GB-A-2.146.638 the lower alcohol ester is hydrolysed in the presence of an acid catalyst, in particular a sulfonic acid and/or sulphuric acid catalyst. Water needed for the hydrolysis of the ester, is supplied to the system from the outside. Unreacted water and the lower alcohol formed upon hydrolysis of the ester are continuously distilled off. After completion of the hydrolysis reaction, the catalyst is separated from the crude fatty acid fraction by dissolution of the catalyst through water washing. Thereafter, the reaction mixture is often subjected to an additional treatment with a carbonate or hydroxide of an alkaline earth metal, to improve the quality of the mixture that is to be subjected to a distillation step, so as to recover the crude fatty acids therefrom.
The process disclosed in GB-A-2.146.638 however presents the disadvantage that the purity of the fatty acids recovered from the distillation step is insufficient. It appears that the fatty acid fraction recovered from the distillation contains impurities that originate from the catalyst.
There is thus a need to find an improved process for direct hydrolysis of fatty esters to the corresponding fatty acids, whereby the fatty acids can be obtained with an improved purity.
This is achieved in the present invention with the features described in the characterising part of the first claim.
In the method of this invention as a catalyst use is made of a compound of a metal capable of forming a.o. with fatty acids, preferably with the fatty acid to be recovered, a soap with a large hydration shell. Such compounds are well known to the man skilled in the art, and include compounds of Al, Ba, Ca, Cd, Co, Cr, Cu, Fe, Li, Mg, Mn, Ni, Pb, Sn, Sr, Ti, Zr, Zn. When adding such a compound or a mixture of two or more compounds to the reaction mixture, the compound is converted to form the catalyst therefrom, in situ. The catalyst is preferably added to the reaction mixture in the form of a compound which is a precursor for the catalyst, so that the catalyst is formed in the reaction mixture in situ.
With the process of the present invention fatty acids can be produced through direct hydrolysis of the corresponding fatty acid ester, in one single step, the amount of soap formed in the course of the reaction being limited.
The fatty acid can be recovered from the reaction mixture in a high purity of at least 99% and is virtually free of impurities originating from the fatty acid ester and the catalyst. The metal concentration appeared to be less than 1 ppm. Because of its high purity, the fatty acid obtainable with the method of this invention is suitable to be used as such in numerous applications where a high purity is an absolute prerequisite.
The fatty acid can be recovered from the reaction mixture by means of a two-pass distillation procedure. In a first distillation pass, unreacted fatty acid ester can be virtually completely distilled off from the reaction mixture. The bottoms of the first distillation pass containing the fatty acid and the catalyst, have been found to be virtually free of unreacted fatty acid ester, alcohol and water. These bottoms are subsequently submitted to a second distillation pass. The fatty acid recovered from the second distillation pass is obtained in high yield and high purity. It appeared namely that the catalyst is non-volatile and remains in the bottoms of the second distillation pass. In this respect it has also been found that, contrary to state of the art, the catalyst of this invention shows an excellent resistance towards decomposition and cracking when it is subjected to higher temperatures in the course of the distillation. As a consequence, the distillation to recover the fatty acid from the reaction mixture can be carried out while the catalyst and high temperature boilers are still present. This is time saving and allows to obtain a process with an improved efficiency. Moreover, the fatty acid recovered from the distillation has been found to be substantially free of cracking residues that may originate from the catalyst. The distillation bottoms containing the catalyst can be fed back to the hydrolysis step and be re-used several times without necessitating an intermediate regeneration or re-activation of the catalyst.
Also, contrary to the process known from the art, there is no need to subject the catalyst to any washing or pre-treatment to destroy the catalyst and allow the fatty acid to be recovered from the reaction mixture. In the process known from the state of the art namely, due to the corrosive nature of the catalyst, the catalyst must be carefully removed from the fatty acid containing fraction by water washing or distillation and the reaction as Well as the distillations must be performed in corrosion resistant materials, which is quite expensive.
The fact that the fatty acid can be recovered without necessitating to destroy the catalyst and the fact that the catalyst can be re-circulated to the reaction mixture as such without necessitating an intermediate regeneration or re-activation, are important economical advantages. In most catalytic processes often the catalyst is responsible for a large part of the costs.
From DE 23213 a process is known for the direct hydrolysis of fats of vegetal or animal origin to the corresponding fatty acids and glycerine by liquid water in the presence of a zinc compound as a catalyst. The process is carried out in an autoclave at a pressure of approximately 9 bar.
In GB-A-617.929 a process is disclosed wherein water and fat am subjected to countercurrent hydrolysis at high temperature, approximately 245xc2x0 C., and high pressure in the presence of a water-insoluble metal soap catalyst. The molten fat is heated and pre-saturated with water by means of high pressure steam. Water is introduced in the reactor by means of a high pressure pump at a rate of about 45 wt. % of the rate of fat feed.
From U.S. Pat. No. 2,511,467 a vapour phase process for converting aliphatic alcohol esters of organic acids to their corresponding acids is known, wherein the ester is vaporised and passed over a catalyst with steam. As a catalyst use is made of a metal salt of an acid corresponding to the acid portion of the ester, for example zinc salts of the organic acids, supported on a suitable carrier material. The process is carried out at high temperature of 250-400xc2x0 C. and a pressure of approximately 3 to 70 bar.
In GB 466 596 a process is disclosed for the continuous countercurrent hydrolysis of a fat to glycerine and the corresponding fatty acids by liquid water at high temperature (between 300xc2x0 F. (149xc2x0 C.) and 600xc2x0 F. (316xc2x0 C.)) and pressure, with or without the presence of a metal soap catalyst such as zinc soap, calcium soap and magnesium soap.
From DE 23 464a process is known for an emulsion hydrolysis of fats to glycerine and the corresponding fatty acids by liquid water at 165xc2x0 C. and in the presence of MgCO3, talc, chalk or thonerde as a catalyst.
In the process of this invention, preferably a catalyst is used which is an oxide, hydroxide, alkoxide or any other water soluble organic salt of the above mentioned metals. Preferably as a catalyst, an oxide, alkoxide or a salt of an organic acid of Zn, Sn, Al, Mg, Ti, Zr, Be, Ca is added to the reaction mixture. By making use of low molecular weight compounds, a relatively large amount of metal can be supplied to the reactor through a relatively small amount of catalyst. Very suitable catalysts are for example acetates or alkoxides of the metals mentioned above.
The compounds mentioned above have been found to show a good solubility in the present reaction mixture, and to be capable of easily forming the catalyst in the reaction mixture in situ. Also, these metals appear to show a high hydration shell when contacted with water, whereby the hydration shell may contain up to six molecules of water or even more. This is advantageous since the solubility of water in fatty acid ester has been found to decrease with increasing chain length of the ester. Typical water saturation concentrations of approximately only 0.1% by weight are found in a fatty acid ester with a chain length of 8 to 10 carbon atoms, which is rather low if an economically favourable reaction rate is aimed at. The hydrolysis of a fatty acid ester should in fact be regarded as a two-phase reaction, one phase containing the fatty acid ester, the other phase containing the water. In this reaction, the contact between the water and the fatty acid ester, and thus the hydrolysis of the fatty acid ester reactant by the water, is mainly restricted to the phase borderline. The catalysts of this invention have been found particularly capable of functioning as phase transfer catalysts. Due to the high degree of hydration of the catalyst, the water concentration in the fatty acid ester phase can be increased, thus allowing the reaction rate to be increased.
In order to accelerate the hydrolysis reaction, the alcohol reaction product is preferably continuously distilled off together with the excess of water. This is preferably performed by conducting steam through the reaction mixture, so as to allow the alcohol formed to be entrained by the steam. Water thus not only functions as a reactant, but also as a carrier for the alcohol formed in the course of the hydrolysis reaction. Also, with this steam stripping, most of the volatile impurities remaining in the reaction mixture, which cannot be removed by a pre-distillation of the fatty acid ester reactant, can be expelled together with the alcohol. Possible examples of such impurities include in particular low molecular weight impurities, for example phenol compounds. The fatty acid ester reactant is namely a natural product, which often contains impurities that may remain in the fatty acid after the hydrolysis and may adversely affect the colour and the odour of the fatty acid. In this way very pure fatty acids can be obtained, especially very pure C6-C10 fatty acids, with a low iodine value, good colour, a good stability towards oxidation because virtually all oxidisable products have been removed. This is particularly important when the fatty acids are to be used in cosmetic applications. The fatty acids obtainable with the present invention appear to show excellent heat stability, thus improving their storability.
The fatty acid ester used in the process of this invention is preferably a methyl ester of the fatty acid. The methanol formed upon hydrolysis of such an ester has a relatively low boiling point and can be easily entrained with the water.
The hydrolysis of the fatty acid ester of this invention is preferably carried out at an elevated, but moderate pressure, preferably between 3 and 20 bar so as to allow the water concentration in the organic ester phase to be increased. Below a pressure of 3 bar the water concentration in the organic ester phase is becoming low, thus involving a decreasing hydrolysis rate.
The process of the present invention presents the advantage that because of the heat stability of the catalyst, the reaction mixture can be distilled at a temperature, which is approximately the reaction temperature, by lowering the pressure to moderate under pressure, for example approximately 300 mbar. There is however no necessity to distil the reaction mixture at extremely low pressure so as to avoid on the one hand cracking or deactivation of the catalyst, and on the other hand contamination of the fatty acid end product by side products originating from undesired side reactions between fatty acid and/or fatty acid ester with the catalyst.
The water concentration in the organic ester phase can also be increased by carrying out the hydrolysis of the fatty acid ester at a temperature of between 150-250xc2x0 C., preferably 180-230xc2x0 C. Below 180xc2x0 C. the reaction rate is getting low. Above 230xc2x0 C. there is a risk that the fatty acid ester is cracked or is withdrawn from the reaction mixture together with the water.
The amount of metal added in the process of this invention is preferably varied between 0.01 to 0.2 percent by weight with respect to the weight of the ester, more preferably between 500 and 2000 ppm. At a concentration below 0.01 percent by weight hardly any catalytic effect can be observed. At a concentration above 0.2 percent by weight no further improvement or acceleration of the hydrolysis reaction could be found.
When producing well defined fatty acid cuts with a pre-determined number of carbon atoms, the fatty acid ester is preferably subjected to a preliminary distillation step, before being subjected to the hydrolysis reaction.
The process of the present invention can be either carried out as a continuous or a discontinuous process.